1 86 PROGRESS OF SCIENCE. 



mination of these types was one of great value to the progress 

 of science. It tended among other things to finally remove 

 the distinction between organic and inorganic chemistry. 

 Gerhardt succeeded in obtaining anhydrous organic acids 

 previously unknown ; showed the true value of vapour volume 

 as a means of determining atomic weight, and developed the 

 idea of series in organic chemistry. 



1817 1892. Hofmann (Aug. Wilhelm) studied the BASES 

 OF COAL-TAR, one of the richest sources of chemical products 

 a fact due to the vegetable origin of the substance, since 

 coal must necessarily contain all the principles and elements 

 of the plants which enter into its composition. Hofmann 

 may be said to have explored a new field, to which the DYE 

 INDUSTRY owes a great part of its development. In his 

 investigations he made many discoveries in the chemistry of 

 the compound ammonias and coal-tar colours ; found the 

 composition and chemical character of aniline red, a decisive 

 discovery, being the earliest of a numerous series in the same 

 sphere. Hofmann's studies of the MOLECULAR CONSTITUTION 

 of the organic bases were so important that he was awarded 

 a Royal medal for them. 



b. 1829. Williams (C. Greville) discovered NEW HYDRO- 

 CARBONS, Isoprene among them, from the distillation of 

 india-rubber and gutta-percha; also alkaloids, such as par- 

 voline, lepidine, cryptidine and what Hofmann called one 

 of the most beautiful substances known to chemists, CYANINE, 

 a superb blue colour, used in astronomical spectrum photo- 

 graphy; investigated beryls and emeralds. Greville Williams 

 established a TABLE OF the atomic weights of the elements 

 in their calculated .positions, showing all THE MISSING 

 ELEMENTS, or gaps, between hydrogen and uranium a 

 work the decisive value of which chemical discoveries will 

 demonstrate in the near future. This Table is an elaboration 

 of MendelejefFs periodic law. 



b. 1832. Crookes, besides his high rank among physi- 

 cists, holds a similar position among chemists. His address 

 as President of the Chemical Section at the British Asso- 

 ciation in 1886, is famous for the bold exposition of his 

 belief in chemical evolution. In his view, one primal ele- 



